This invention relates to an apparatus for calibrating a pyrometer for measuring temperatures in a furnace and, more particularly, to an apparatus for calibrating a pyrometer for measuring the surface temperature of furnace tubes.
Two types of pyrometers that are used to measure the temperature in process furnaces are the optical and infrared radiation pyrometers. In the optical type a small wire, which is heated by a calibrated variable current, is superimposed on the image of the object to be measured. The current provided to the wire is varied until the color of the wire matches the color of the object to be measured. The current is then read from a dial calibrated in temperature units. The infrared type of pyrometer gathers radiation from the object of interest and focuses it on an element which converts the radiation to a current or voltage signal. This signal, which can be biased to compenate for the emissivity of the object to be measured, is amplified and provided to a meter. Although such pyrometers provide an improvement over the prior art temperature measurement techniques, it has been found that the accuracy available with such units is unsuitable for certain applications. For example, in a pyrolysis furnace, measurement of the temperature of the surface of the furnace tubes is used to detect the presence of hot spots caused by coke formations or other residue inside the tube. These formations decrease the heat transfer of the tube wall, thus allowing the temperature of the tube to approach that of the furnace wall. Heating of the tube beyond its tolerance limits can cause it to rupture. Generally, the most efficient operation of the furnace requires that the furnace tubes be operated near their maximum temperature limit which increases the possibility of overheating and rupturing of a tube. However, to prevent any possibility of damage to the furnace prior art workers have typically provided a margin of safety by operating the furnace at less than maximum temperature to allow for the inaccuracies encountered with the prior art pyrometers, thereby reducing the efficiency and productivity of the furnace.
Therefore, it is an object of the present invention to provide an apparatus for calibrating a pyrometer for measuring the temperature of a furnace tube or other body in a furnace.